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Old 01-05-2009, 07:15 PM   #1
Back Road Runner
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Default Intake idea, adjusting runner cross-sectional area and length

I've been slowly tweaking my car here and there for quite some time. I've kind of look at the intake as one area that has been largely overlooked (well, the right area has been overlooked), the manifold.

In a NA engine, the intake and exhaust manifolds work exactly the same way, both for flow and tuning. Air temps and pressure differences are a bit different, but the design works the same way. Effectively, the intake runners and exhaust primaries do the same thing. As well, the intake plenum and exhaust secondary collector do the same. This is my thinking anyways.

You change the diameter of the piping, changing cross-sectional area to change where the peak torque is located. If the diameter is larger, the peak torque point is raised up the power band. If the diameter is smaller, the peak torque moves down the power band.

You change the length of the piping to tilt the curve above or below this peak point. If you run a short length pipe, you tilt the curve towards the higher rpms. If you run a long length pipe, you tilt the curve towards the lower rpms.

Between these two, you can define a peak point and pick a focus area for the power band.

Now everything is a matter of trade-offs, and you can have everything all at once. The key then is to build towards an intended goal.

My personal goal has been much more low and mid range torque. This would automatically point me towards smaller pipe diameters and longer pipe lengths. But again, there can be compromises. You can pick a high peak but focus low, pick a low peak and focus high, and so on.

So far, I have done very little with my car. The only major mods to my engine is an ECU flash, throttle body spacer, and 8mm intake manifold spacer.

The intake manifold spacer was especially interesting because it actually did quite a lot in terms of changing the power band and quite noticeably improved low end torque. However, it also noticeably but lightly dropped high end torque. The changes were significant enough to intrigue me about modifying my intake again. I plan to try a second 8mm spacer on the manifold to further bump up runner length. However, I am already seeing drops in top end power. Along with the gain in length, I plan to have some porting done on the intake to bump up diameter as well as improve flow. This would bump up the peak torque point to offset the top end drop and the length would give me strong low end focus. Since the redline is only around 6500 rpm, that's kind of all I need to maintain up to.

Looking at the exhaust, you will see most of the aftermarket headers are geared similarly. Both the OBX/Rallitek/Cobb triplets and TWE use basically a 1.625" primary pipe diameter and a length of right around 40". For our cars one can use a 1.5" or 1.625" diameter primary, geared for mid or geared for upper end respectively. One could go larger if they intended to build a race spec engine. The pipe length for midrange power is more towards 36", towards 32" for high end focus, and towards 39" for low end focus. You can kind of see both header designs focused towards high end peak but with heavy low end focus to keep solid low and midrange torque.

This can be mimicked with the intake manifold too.

Now look at my dyno runs.
http://www.gigafiles.co.uk/files/185...torque%202.jpg

Focus on the torque curve. Look at both the shape and tilt of the curve. The dotted line is an old dyno run I did on a Dyno Dynamics. The solid red and blue lines are from a series of road dynos I did after the spacer install. Now diameter hasn't changed, but length has. Length changes the focus of the curve, the tilt. In stock form, we can see that the tilt is a bit towards the upper end, so you get a midrange focus from the diameter and a slight tilt towards the upper end for high rev power. Once the spacer was added, the curve is basically symetrical above and below with no tilt. If I added a second spacer, I would tilt it towards the low end some. If I change the diameter (porting), I would move the peak up, and the tilt would be relative to that. What I'm kind of looking for is a little higher peak, but with a heavy low end tilt. Porting and adding a second spacer should do this. Now the downside is that power on the top would drop like a rock. However, I've got a redline of 6500 rpm, and I'm not looking to raise that. As well, I'm not running at 6500 rpm 99% of the time. It's kind of the idea of if you're not going to use it anyways, why have it. On the flipside, you should be able to see that the stock intake, just ported would work well for someone building a high rpm setup because it would lean towards the high end plus with the porting would also peak high.

Anywho, that's my thinking. It will be a little while till I actually do anything with my car, but I thought I'd share some ideas with you guys to get you thinking about your own cars some. Most folks think about the intake in a completely different and far less useful light with the CAIs and such. It's the same with the exhaust and looking only at cat backs. Most of what goes on happens in the manifolds. The CAIs and cat backs are nice for flow, but they simply don't address the main areas.
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Old 01-05-2009, 07:41 PM   #2
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Interested to see what an extra 8mm spacer will do????
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Old 01-05-2009, 08:00 PM   #3
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I think this belongs in the built motor section, though I know it'd be lost on the boOst junkies.

Interesting ideas.......

Jay
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Old 01-05-2009, 08:16 PM   #4
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Well, it's not really a "built motor," and the installation of hardware is no more then a CAI or anything else. As well, boosted folks don't care so much. Forced induction doesn't really work on the same principles. NA is the only setup that makes use of flow velocity and pressure wave tuning.

For those that aren't familiar, the reason that pipe diameter and pipe length is important is because NA engines and the flow of air/fuel through the engine and the efficiency of this process depends upon the velocity and pressure waves (like musical instruments) to get the job done efficiently. This is how we can adjust peak torque as well as the shape or lean of the torque curve by messing around with pipe diameter and length. There are other factors that play, but this is the brunt of the external appendages we can play with to change what happens inside the engine.
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Old 01-05-2009, 08:19 PM   #5
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Quote:
Originally Posted by christianvwcars View Post
Interested to see what an extra 8mm spacer will do????
Yes. The one made quite a noticeable difference. I would like to test the additional change of adding more then one. The hard part is that there are various pipes and brackets that attach to the intake manifold, and they are sort of only so long. At some point, you might not fit something back on or would have to fabricate an extension to fit something. As well, it moves the intake manifold and air box towards the hood of the car too which could eventually hit.
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Old 01-06-2009, 01:25 AM   #6
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Do you happen to have before and after engine load graphs?
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Old 01-06-2009, 02:37 PM   #7
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No. At the time, I wasn't really compelled to go through the physical work. This spring, I probably will though as I want a little more detail on the variations. All I'm going with right now is my old dyno run and my own perceptions. The intake manifold spacer made a relatively noticeable difference. The graphs correlate pretty well what I feel, so I deem them at least adequately valid, maybe not so much for raw numbers for valid enough for relative comparison and concept.

Come spring time, I will probably spend a weekend stepping back to stock, testing, and going back through the mods I've done. Current is mild and reversible: I-speed reflash, throttle body spacer, intake manifold spacer. I can pretty easily step back to stock in all regards, even have a spare ECU with a stock map.

I've done no engine load mapping, just logging rpm and tossing it in Data Log Lab.

Last edited by Back Road Runner; 01-06-2009 at 02:42 PM.
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Old 01-06-2009, 08:00 PM   #8
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ill be looking forward to any data you compile on the stock vs. spacer vs 2 spacers. this is something that i am curious to see actual load/dyno #'s on. Will 2 spacers fit without modifying Vac/emissions plumbing to the intake manifold? I got the impression that one spacer was almost too much for the OEM plumbing.
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Old 01-06-2009, 08:20 PM   #9
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On the newer cars, 1 spacer won't even fit without modifying the EGR tube.
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Old 01-06-2009, 09:33 PM   #10
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hmm I was also thinking of putting the phelonic spacers plus a TB spacer on my EJ22 but my car is MAF equiped so i'm not sure how it would affect the readings.
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Old 01-06-2009, 10:00 PM   #11
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You should be fine. It would be interesting to see how much you can change the intake runner length before you change the resonant frequency so much that the Helmholtz resonator in the fender is no longer effective and you get the problem I laid out in the CAI/SRI paper.
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Old 01-06-2009, 10:12 PM   #12
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would lengthening the runner length increase or decrease the natural resonant frequency? i want to say decrease but my reasoning is not solid.
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Old 01-06-2009, 10:30 PM   #13
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I vote for williaty to do a test with it
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Old 01-06-2009, 10:38 PM   #14
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Quote:
Originally Posted by williaty View Post
On the newer cars, 1 spacer won't even fit without modifying the EGR tube.
Mine's not really new, and it's a 1998. Yes, i had to lengthen the EGR tube
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Old 01-06-2009, 10:49 PM   #15
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Quote:
Originally Posted by RaceFaceXC View Post
would lengthening the runner length increase or decrease the natural resonant frequency? i want to say decrease but my reasoning is not solid.
Decrease, yes. This is why I don't think it would be a problem as pushing the resonant frequency below the effective range of the Helmholtz resonator stands a very good chance of pushing it below idle.

Quote:
Originally Posted by Zac86 View Post
I vote for williaty to do a test with it
I really would like to. I talked to GrimmSpeed about it and they offered me a little bit of a deal. However, by the time you add the spacers, gaskets, EGR extension, etc all together, I couldn't afford the bottom line. I was sad

Quote:
Originally Posted by 2.5GTLegacy View Post
Mine's not really new, and it's a 1998. Yes, i had to lengthen the EGR tube
Huh, so why doesn't someone, say the vendor making the spacers, make an extended EGR tube?
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Old 01-06-2009, 10:51 PM   #16
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im a yamaha tech and we have had variable length velocity stacks for a couple years now. it works great because you get the best of both worlds. at low rpms the stacks are about 4 inches long and at high rpms the top 3 inches of the stack lift off the bottum stack and leave you with a short 1 inch stack. it was the only way they could get some bottum end out of a 600cc inline that revs 16500 rpms.
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Old 01-06-2009, 10:53 PM   #17
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my 98 ej22 doesnt have EGR
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Old 01-07-2009, 04:39 AM   #18
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Variable runners would be cool, but it would have to be an aftermarket piece. I know a variety of cars run dual runner setups to gain low + high rpm functionality, but Subaru has yet to incorporate such a design. Now, it's not exactly needed as these engines do offer quite a wide power band that doesn't exactly necessitate the need for such changes. The concept just isn't new.

On my car, I had no issues with hardware not fitting. I think the worst parts were a bracket on the right side (passenger) that screwed onto the manifold and a hose that went to the stock air box that is barely long enough. I'm not sure what the fitment issues would be for adding another spacer. It seems some folks run into different issues then what I ran into with my car. It may involve buying some new hose or extending a bracket a little (JB Weld + some sheet metal? ).

I think these are kind of expected issues when changing the physical dimensions of a system that was initially designed to be relatively specific.

The throttle body spacer is an interesting side note. My understanding is that the plenum area of the intake is proportional to the volume of air of the runner. My infant understanding of this is that the size kind of determines engine behavior. If small, the engine will be more reactive to throttle inputs but also rougher in behavior as the air needs to be pulled more immediately from the intake track. A larger plenum volume creates a larger reservoir for the runners to pull air from, smoothing out power delivery but also creating more of a delayed input to throttle actions. A throttle body spacer adds to this plenum volume.

One thing I noticed when I added my throttle body spacer, prior to the intake manifold spacer, was how much smoother power delivery became. The engine ran quite smooth and even as it reved up. In stock form, it ran slightly rough. Once I added the intake manifold spacer, the engine behavior got slightly rough again, just like stock. There seems to be a definite correlation to both parts, although, I don't know the exact numbers nor where optimum may be. For example, with the intake manifold spacer, I would venture to guess not running a throttle body spacer might actually be slightly detrimental to engine behavior in the sense that it could operate roughly. Heck, I wouldn't mind trying a second throttle body spacer too, but that does tend to get a little rediculous. I'm not even sure if the stock air box would fit afterwards or if it would be shoved against the firewall by this time, or into the hood (with a second intake manifold spacer). The plenum volume is something to think about when adjusting the runners. The throttle body spacer itself really did nothing for power. All it really did was smooth out what was already there. The intake manifold spacer was the only thing that actually changed the amount of power and where. I want this to be clear for the folks reading this. As well I want to be clear that both mods did not noticeably raise overall peak power but rather change the shape of the power over the rev band, adding more in some areas and decreasing in others. Peak power is still a matter of the combustion process and compression or temps have to change to actually make more raw power. Sometimes overall gains can be had through improving efficiencies in areas that are severely lacking, like the exhaust and adding aftermarket headers. The intake isn't really inefficient though, and I think most of what you can do is simply work with the shape of power delivery, not increasing peak power, at least not like we see from the exhaust. You folks with the newer cars with stock equal length headers don't see as big of gains in the exhaust as we older car folks see because of this too.

Enough babbling for now.
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Old 01-07-2009, 11:08 PM   #19
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i have beeen saying that grimmspeed should make that egr pipe . its total b.s.

so are you guys saying that because i rock the group a throttle body spacer and the grimmspeed 8mm spacers, that i now have no issue with the voltage spike maf issues?????

because im rollin with a custom sri and it feels awesome . i cant wait to port and polish the heads , then ill have some real volume and flow working
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Old 01-07-2009, 11:11 PM   #20
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Quote:
Originally Posted by watchunglava View Post
so are you guys saying that because i rock the group a throttle body spacer and the grimmspeed 8mm spacers, that i now have no issue with the voltage spike maf issues?
Not at all. I'm saying that you've definitely made changes to the intake tract that may make it behave differently than a fully stock intake tract. Without careful testing, there's no way to know if you made sufficient difference to avoid resonance issues or not.

FWIW, just the TBS alone is not enough to save you. I've got one.
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Old 01-07-2009, 11:15 PM   #21
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wility you should definatly do the 8mm spacers though they really open up the low end . i did both of those mods right but at different times. the tbs kinda helped out the high end where the spacers really kick it up down low . ( total butt dyno but whatever thats how it was)
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Old 01-07-2009, 11:29 PM   #22
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While I would like to end up with considerably more torque below 4.4kRPM, I don't want to sacrifice it between 4.kRPM and 5.5kRPM because the car has so little up there to begin with. From BRR's plots, it looks like the spacer starts showing a torque loss at about 4.7-4.8kRPM.
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Old 01-08-2009, 12:19 AM   #23
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Well, yeah. As with all things, it's a trade off. I see myself using low and midrange torque so much more then any high end, so I favor the spacer. You are right to be concerned about high rpm loss. That is a concern myself, especially if I try two spacers. Power may drop like a rock above 5k. However, I see porting as the savior to this as it will bump up the peak point. If standard porting and polish gets another 500 or even a 1k rpm bump, depending on how drastic, one may not even see any power loss pretty much up to redline. That would be cool.

Now I want everyone to know I am running a stock exhaust and stock cams. The exhaust would also do similar work as the intake and can be tuned similarly. The cams will also play a roll in power band and influence. It's good not to get too focused on just one aspect. An engine is a system, and the end result is a whole of many small parts. I focus on intake, mainly the manifold, because it is so commonly ignored. I don't think 99% of the people here have even considered the intake manifold as a tuning tool. Sure, some folks port and polish, but they don't really do it for a specific reason or end goal. Just like a lot of people tossing on CAIs, they simply think "more flow = more power." To a basic extent, yes, but in the grand scheme, not necessarily. It's like tossing on a 4" exhaust, you can cause detriment if you don't do it right.
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Old 01-08-2009, 01:51 AM   #24
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I started a thread that's pretty closely related to this but provides a more me-centric spin on things. I have a feeling that most of the people interested in this thread will also be interested in mine, but I didn't want to clog up BRR's thread with nonsense
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Old 01-08-2009, 11:56 AM   #25
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Quote:
Originally Posted by Back Road Runner View Post
...a variety of cars run dual runner setups to gain low + high rpm functionality, but Subaru has yet to incorporate such a design.
See: IRIS valve, 3.3L. The SVX H-6 intake manifold initially acts as a pair of 3-cylinder manifolds, opening the valve for a large plenum at higher RPM.

One of the things that seems to be overlooked anytime someone mentions altered or fully custom intake manifolds - the math behind the runners. Lots of people sling around the term "ram air" just like GM did, meaning that a pressure area stuffs air into the pre-TB intake, and that's swell, but there's another meaning which is much more important. The speed and frequency of the air pulses in the intake runners defines "ram air". Calculations made there might hold more merit than simply throwing parts and seeing what happens.

I'm looking forward to your results, either way.
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